1. Field of the Invention
The present invention relates generally to an apparatus for detecting a focus position and, more particularly, to an automatic focus control apparatus that is for use with, for example, an auto-focus apparatus for a television camera and the like.
2. Description of the Prior Art
Auto-focus apparatus that detect a properly-focused position of a lens by detecting the contrast of a picked-up image are known (see Japanese Patent Application No. 62-146628). Varying the position of the lens while an object is picked up by the lens, it is to be noted that the picked-up image provides a best contrast at the position in which the object is kept in focus. For simplicity, this focus position will hereinafter be referred to as a properly-focused position.
Accordingly, as shown in FIG. 1, a properly-focused position P of an object can be detected by detecting the position of the lens at which an output signal from an image pickup element provides a maximal amplitude value.
According to a television camera based on the above-mentioned principle for measuring the amplitude value, the lens is moved away from its position closest to the object to sequentially detect the amplitude values of the output signals, whereby the television camera can capture the properly-focused position P by detecting the position at which the switchover from the increase to the decrease takes place in the change of the amplitude value. Therefore, by adjusting the position of the lens on the basis of the detected result, it is possible to properly focus the predetermined object.
Further, in the television camera, once the properly-focused state is established, the position of the lens is vibrated back and forth around the properly-focused position P by a predetermined width P.sub.W (hereinafter, this vibrating movement is referred to as "wobble") so that any change of the properly-focused position P can be detected regardless of the change of the properly-focused position P due to the movement of the object. Thus, the position of the lens is adjusted on the basis of the detected result, thus making it possible to provide the properly-focused state following movement of the object.
This kind of auto-focus apparatus, however, has a defect in that the quality of a picked-up image is inevitably deteriorated. Because the position of the lens is wobbled around the properly-focused position P by the predetermined width P.sub.W, the picked-up image is also wobbled unnaturally, resulting in the quality of the picked-up image being deteriorated.
As one of the methods for solving the above problem, a method for wobbling the image pickup element instead of the position of the lens is known. According to this previously-proposed method, although the picked-up image can be prevented from being unnaturally wobbled and the properly-focused condition can be brought about, it needs a driving mechanism such as an actuator or the like that permits the image pickup element to wobble, in which case the overall arrangement of the auto-focus apparatus becomes complicated.
Moreover, since the image pickup element is wobbled as described above, the picked-up image is caused to be out of focus repeatedly at a predetermined period, thus the image quality thereof is deteriorated. Accordingly, the image pickup element has to be wobbled at such a low frequency as to prevent the deteriorated quality of picked-up image from becoming conspicuous to a viewer. When the moving speed of the object is too high, it becomes difficult to provide a properly-focused condition.